In a jetting device the medium to be jetted is first transported, or fed, to a chamber, and then ejected from the chamber through a nozzle. Thus a droplet of the medium leaves the device and, typically, impacts a closely arranged surface and thereby forms a dot on the surface. The operation of feeding the medium to the chamber is of great influence for the performance of the device, for example as regards the jetted amount, i.e. the volume of the droplet.
A prior art device showing an overall good performance is disclosed in WO 99/64167, where the principal parts of the feeder are a tubular portion defined by a plurality of O-rings, which are disposed in a cylindrical boring, and which are stacked on top of each other; and a threaded shaft that extends through the tubular portion. At an upper end of the thread there is provided an inlet for the viscous medium, and at a lower end thereof there is provided an outlet. The outlet communicates with a chamber of an eject mechanism, and said chamber opens at a nozzle. In order to fill the chamber with the medium, the shaft, which could be regarded as a screw, is rotated, for example for a fraction of a turn, whereby the medium is forced out of the outlet and into the chamber. The eject mechanism then pushes the medium out through the nozzle by rapidly and temporarily decreasing the volume of the chamber.
However, due to small dimensions, the screw is difficult to manufacture with high precision, and the stack of O-rings, while providing a good flexible seal towards the thread of the screw, is not optimal as regards the transport of the medium through the feeder, since some amounts of the medium tend to get stuck at a bottom area of the narrow recesses formed between adjacent O-rings.
There is, however, another type of device which comprises a feeder that does not suffer from the problems of the feeding screw. The device is an extruder as shown in the published German patent application DE 2 649 045. The feeder of the extruder comprises a cylindrical shaft and tubular portion having an internal thread. When the cylindrical shaft is rotated the plastic granule is dragged along in the rotation. The thread guides and redirects the rotational movement of the plastic granule into a movement along the thread, and, thus, the granule is fed forward through the feeder.
This prior art feeder works well in an extruder, and it would be desirable to use the same feeder principle in a jetting device. However, the extruder feeder has a clearance or gap between the wall of the feeder tube and the shaft. The gap is necessary in order to keep the heat increase due to friction on an acceptable level, but results in a problem of leakage when it comes to the viscous mediums that are used in a jetting device. If the principle structure of the prior art extruder feeder would be applied to a feeder of a jetting device, there would be a problem of obtaining an equal droplet size. When the shaft would be rotated in order to feed an amount of the medium into the eject chamber, a pressure is generated, which would in turn force an amount of the viscous medium back through the gap. Due to the small dimensions of the feeder and small volume of each droplet, an arbitrary or not fully controllable variation of the droplet size introduces an undesired problem.
In addition, it would not be possible to eliminate the gap, since then the shaft would not be rotatable.